Packages formed with films are manufactured by a good number of methods which utilize to advantage the characteristics of the films such as the bag sealing method, twist wrapping method, thermal shrink wrapping method, cohesive wrapping method by use of specific films represented by Saran Wrap (product by Asahi-Dow Limited), stretch wrapping method and the like. These methods require respective wrapping characteristics. For each packaging method, therefore, it is important to select a film whose basic material, composition, form and characteristic attributes best suit the wrapping characteristics of the particular method employed.
Of these packaging methods, this invention primarily aims to provide a film particularly suitable for the shrink wrapping method. Nevertheless the film of the present invention need not limit its uses but may be put to other uses satisfactorily. Thus, this is a unique multipurpose film never attained t o date. For the convenience of illustration, therefore, the present invention will be hereinafter described with reference to the film formulated t suit the shrink wrapping method.
Generally, the shrink wrapping method effects required wrapping by virtue of the thermal shrinkability of a film stretched and oriented in fixed directions, specifically resorting to a procedure of loosely prepackaging a given commodity with the film as by sealing, and thereafter thermally shrinking the film as enclosing the commodity therein by means of a suitable heat medium such as the hot air, infrared ray or hot water for thereby causing the film to shrink and come into skintight contact with the overall irregular contour of the commodity. This method is characterized by providing a package which has a beautiful appearance, imparts an enhanced commercial value to the content, keeps the content in a hygienic condition and yet permits the content to be examined for its quality through visual observation or by sensation of touch. This method enables even a commodity of irregular shape or a plurality of commodities to be packaged with ample tightness in a single piece and provides the content with effective protection against vibrations and other impacts.
Further, the shrink wrapping method provides speedy packaging as compared with the stretch wrapping method which is extensively used such as in super markets. As an effective method for industrial packaging of heavy articles of large dimensions which the stretch wrapping method is incapable of packaging, this shrink wrapping method is finding rapidly increasing acceptance and arresting keen attention.
Moreover, it permits packaging of commodities of shapes so irregular as to defy effective packaging by the stretching method and enables desired packaging to be accomplished without use of trays or other containers. It also enjoys greater tightness of package. In spite of all these advantages, the shrink wrapping method has a disadvantage that the package must be amply heated until the film shrinks to required tightness.
It is the oriented film of plasticized polyvinyl chloride (hereinafter referred to as PVC) that is now used most widely for the shrink wrapping. This is ascribable to the film's great merit of readily undergoing thermal shrinkage of a high rate of relatively low temperatures and providing satisfactory shrink wrapping in a wide range of temperatures. On the other hand, this film nevertheless has a disadvantage that it provides heat sealability, preservability (liability of the plasticizer to degrade properties brought about by the orientation of film) and moistureproofness less than normally required, entails a hygienic hazard due to use of the plasticizer, emits noxious gases such as chlorine-based gases when the film is cut by means of heated wire, issues corrosive gases when the film, after use, is burnt in an incinerator and, because of its inferior cold resistance, tends to rigidify, embrittle and rupture when the packages using the film are stored at low temperatures or handled in cold districts.
In recent years, therefore, increasing attention has come to be focussed on a polypropylene type (hereinafter referred to as PP) film for use in the shrink wrapping method. The PP film has a disadvantage that it provides inferior shrinkability to the PVC film. The oriented film of the PP type is excellent in mechanical property, moistureproofness, heat seal strength, heat resistance and film modulus and, therefore, proves to be highly suitable for use as a film for the shrink wrapping.
Further, PP is advantageous over PVC in terms of raw material cost and because of low specific gravity. Because PP is a rigid, crystalline polymer possessing a high softening point, the PP film requires heating at a higher temperature for necessary shrinkage than the conventional oriented films and exhibits a very slight degree of shrinkage at low temperatures in the neighborhood of 100.degree. C. Thus, the PP film must be heated at high temperatures in the course of the shrink wrapping. Moreover since the allowable range of temperatures for the heating is narrow and the dependency of the rate of shrinkage upon temperature is heavy, a locally uneven heating possibly given to the film at the time of wrapping results in a notable uneven shrinkage which tends to cause creases, dots resembling pockmarks and other surface irregularities which are undesirable from the viewpoint of practical use of film. More heating given to the film for the purpose of preventing such uneven shrinkage brings about a serious drawback that the content being packaged is excessively heated, the film is deprived of its transparency, and the film is ruptured along the sealed portion and around the air vents. Generally, the PP film is available preponderantly in small thickness. If the thickness is increased, the PP film becomes too rigid to permit successful shrink wrapping and tends to sustain rupture readily.
The conventional low-density polyethylene (hereinafter referred to as LDPE) film in its unaltered form does not permit sufficient orientation of molecules. The oriented LDPE film resulting from the treatment of stretching, therefore, exhibits low thermal shrinkage and particularly low thermal shrink tension, requires high temperature for shrinkage, offers poor film strength and optical property, produces low binding force in the package with respect to the article contained. Because of these inferior properties, the LDPE film which is produced in an increased thickness is put to special uses.
In the case of LDPE film, if the film is stretched thoroughly at a high temperature exceeding its melting point by use of a high-energy ray so as to cause crosslinking of molecules, the oriented film enjoys high processibility, permits required temperature to be set effectively in a range of high temperatures, exhibits high thermal shrinkage and high thermal shrink tension and excels the LDPE film in various properties such as optical properties including transparency and gloss, resistance to heat and the like. In the range of high temperatures, however, the thermal shrinkage is not high enough to permit effective heat sealing and the film strength is degraded to impair the heat sealability and tear resistance.
Further, the oriented LDPE film has a disadvantage that the cutting and the sealing of film by means of a heating wire are difficult to effect, the physical properties, particularly the optical properties are degraded subsequently to the thermal shrinkage, the film strength is lowered, and the film tends to sustain rupture and creases around air vents at the time of shrink wrapping. Because of these drawbacks, the shrink wrapping by use of the oriented LDPE film is inferior in terms of speediness of operation and finish.
As is clear from the foregoing description, one important requirement for successful shrink wrapping resides in the fact that the film should permit required packaging to be effectively carried out at low temperatures. This requirement is particularly significant when the packaging is given to fresh food.
The oriented PP film is produced by a procedure of extruding the molten polymer through an annular die into a tubular raw film, suddenly cooling the extruded tubular raw film, again heating the raw film at high temperatures in the range of from 150.degree. to 160.degree. C. and simultaneously introducing air into the interior of tubular raw film. The oriented LDPE film can be produced by the conventional procedure which is employed in biaxially stretching. These processes are extremely difficult to accomplish from the technical point of view because the films are highly liable to sustain rupture.
Thus, the direct inflation method which involves a procedure of extruding the molten polymer at a temperature in the range of from 180.degree. to 220.degree. C., for example, and thereafter suitably cooling the extruded tubular film with ambient air and, at the same time, inflating the film to a film of a desired size is generally employed.
The inflation method is characterized by being capable of producing a desired film readily and inexpensively. It nevertheless suffers from a disadvantage that the treatment entails irregular flow and crystallization of molecules and impairs the optical properties of film and the stretching fails to provide satisfactory setting of molecular orientation. Consequently, the thermal shrinkage and thermal shrink tension are deficient and high temperatures are required for ensuring their sufficiency. The film produced by this method, therefore, in impracticable unless it is produced in an increased and put to special uses. To overcome the disadvantage, there have been developed improved methods resorting inveriably to a procedure of extruding LDPE in the form of a tubular film, exposing the film to a high-energy radiant ray under suitable conditions for thereby inducing a partial crosslinking reaction in the film and reheating and stretching the film so as to effect required setting of molecular arrangement sufficiently without entailing random intermolecular flow. The conventional inflation method, however, produces a film which is not free from the aforementioned drawbacks.
A good many methods have heretofore been suggested for producing films by mixing polymers of different olefins or mixing polyolefins with other polymers and subsequently inflating the resultant blends. For example, U.S. Pat. No. 3,682,767 discloses a method for producing a film possessing improved melt strength and heat sealaibility and exhibiting improved make-and-fill property at the time of packaging a liquid commodity by a procedure of mixing ethylene, an olefin type unsaturated monomer such as, for example, ethylene-vinyl acetate copolymer (hereinafter referred to as EVA) and a linear copolymer of ethylene with an .alpha.-olefin having a density in the range of from 0.93 to 0.96 l g/cm.sup.3, such as, for example, a modified high-density polyethylene (hereinafter referred to as HDPE) and subsequently extruding the resultant blend in the form of a flat or tubular sheet. British Pat. No. 988,299 teaches a process for producing a printable polyethylene film by a procedure of mixing EVA eith LDPE or HDPE, causing crosslinking in the resultant blend either before or after molding, and subsequently stretching the blend in the form of film. And British Pat. No. 1,035,887 concerns a process for the production of a film excelling in low-temperature properties by a procedure of mixing LDPE with a linear medium-density polyethylene obtained by modifying ethylene with a small amount of butene and stretching the resultant blend.
As to manufacture of films, British Pat. No. 998,299 mentioned above involves a procedure of treating the aforementioned composition with a peroxide or a high-energy ray and thereby causing crosslinking and subsequently stretching the crosslinked sheet at temperatures close to or slightly higher than the melting point of polyethylene and British Pat. No. 992,897 adopts a procedure of treating EVA with a high-energy ray and thereby causing crosslinking and subsequently stretching the crosslinked sheet at elevated temperatures (preferably in the range of from 100.degree. to 120.degree. C., for example). The films obtained of such compositions are devoid of the excellent optical properties, strength properties and low-temperature shrinking property enjoyed by the aforementioned PVC type films and they fail to exhibit satisfactory film-forming property.